Attention has been drawn to an optical multilevel modulation method in aiming for constructing a super-high-speed optical transmission system of 100 Gbit/s or higher. In particular, a coherent receiving method such as DP-QPSK (Dual Polarization Quadrature Phase-Shift Keying) has attracted attention because of its advantages of enhanced optical noise immunity and compensation performance by electrical signal processing on wavelength dispersion distortion after photoelectric conversion. Application of the coherent receiving method to a transmission system has been actively discussed. An optical receiver used in the coherent receiving method includes: a local oscillation light generator configured to generate a local oscillation light beam; a polarized beam splitter configured to separate a signal light beam and the local oscillation light beam for output to different output ports according to the polarized wave states; an optical 90-degree hybrid circuit configured to multiplex the signal light beam and the local oscillation light beam; a photoelectric converter configured to convert an output signal from the optical 90-degree hybrid circuit to an electrical signal; an AD converter configured to convert the electrical signal outputted from the photoelectric converter to a digital signal; and a digital signal processing (DSP) circuit configured to perform calculation using the digital signal. The optical receiver separately detects an in-phase component and a quadrature component of an interfering light beam of the inputted signal light beam and local oscillation light beam, and thereby can obtain information included in the inputted signal light beam.
As one of the components of the optical receiver used in the coherent receiving method, an optical 90-degree hybrid including a spatial optical system having combined bulk-type optics has been developed and commercialized. Meanwhile, a planer lightwave circuit (PLC) including optical waveguides formed on a planer substrate is superior in mass-productivity and reliability to the aforementioned spatial system. Moreover, employing the PLC-type optical 90-degree hybrid circuit gives a higher possibility of achieving integration of, for example, the polarized wave beam splitter and the photoelectric converter and enables provision of a more compact optical receiver than employing the spatial optical system. Because of such a background, commercialization of the PLC-type optical 90-degree hybrid circuit is expected.
FIG. 1 is a configuration diagram showing a conventional optical 90-degree hybrid circuit. The conventional PLC-type optical 90-degree hybrid circuit is shown in Patent Literature 1. Patent Literature 1 relates to an optical delay interference circuit used for demodulating a DQPSK (Differential Quadrature Phase-Shift Keying) signal. The optical delay interference circuit itself is not a part forming the optical receiver used in the coherent receiving method, but has, in apart of the circuit, a function as the optical 90-degree hybrid circuit configured to multiplex the two light waves and separate the light waves to the in-phase component and the quadrature component. Hereinafter, the in-phase component is expressed as an “I component,” and the quadrature component is expressed as a “Q component.” FIG. 1 shows a configuration of only a circuit portion required to achieve the optical 90-degree hybrid function, which is extracted from optical circuits described in Patent Literature 1.
Here, a description is given of a propagation process of light beams inputted to the conventional PLC-type optical 90-degree hybrid circuit in FIG. 1. A signal light beam inputted from the outside of the PLC passes through an input waveguide 1a and is split into two by an optical splitter 2a. A local oscillation light beam inputted from the outside of the PLC passes through an input waveguide 1b and is split into two by an optical splitter 2b. The two light beams split by the optical splitter 2a are inputted to two optical couplers 3a and 3b through arm waveguides 10a and 10b, respectively. The two light beams split by the optical splitter 2b are inputted to the two optical couplers 3a and 3b through arm waveguides 10c and 10d, respectively. The signal light beam and the local oscillation light beam inputted to each of the optical coupler 3a and the optical coupler 3b are multiplexed to form an interfering light beam. The interfering light beam is split into two interfering light beams having a phase difference of 180 degrees, which are then outputted. The interfering light beams, of the signal light beam and the local oscillation light beam outputted from the optical coupler 3a are outputted through output waveguides 4a and 5a to a differential receiver 6a formed as an external circuit and functioning as a photoelectric converter. The interfering light beams, of the signal light beam and the local oscillation light beam outputted from the optical coupler 3b are outputted through output waveguides 4b and 5b to a differential receiver 6b formed as an external circuit and functioning as a photoelectric converter.
Any one of the four arm waveguides 10a, 10b, 10c, and 10d is provided with a 90-degree phase shifter 7. Thereby, the differential receivers 6a and 6b perform differential detection on the interfering light beams outputted from the optical coupler 3a and the optical coupler 3b through the output waveguides 4a, 4b, 5a, and 5b, so that the I component and the Q component of inputted modulated signals can be separated. Meanwhile, in order to simultaneously detect the I component and the Q component of the modulated signals, the two arm waveguides 10a and 10b configured to transmit the signal light beams split by the optical splitter 2a, and the two arm waveguides 10c and 10d configured to transmit the local oscillation light split by the optical splitter 2b need to have an equal waveguide length except for the shifter 7. Further, when the four arm waveguides 10a, 10b, 10c, and 10 are formed to have the equal wavelength except for the shifter 7, the circuit can be utilized as an optical 90-degree hybrid circuit included in the optical delay interference circuit for receiving differential phase modulated signals of DQPSK or the like.